This board provides 8 optically isolated inputs that accept a wide range of voltage, including the 3VDC and 5VDC signals available on the Arduino. This is a great way to switch on or off AC or DC loads using your Arduino!
The relays are single-pole double-throw (SPDT) and are rated for 10A @ 250VAC. The ground for the relay coils is separate from the ground for the input signals. This, along with the optical isolation, provides a total galvanic isolation between the input and output. The two grounds could be joined together if desired with the included jumper.
The relay coils run on 12VDC, and the module has screw-terminal blocks for sturdy and reusable attachments for the external load. The signal and power inputs run through the male single-row header.
Here is a schematic diagram of this board. This diagram shows one of the relay channels; the board contains eight duplicates of this circuit.